An earth-boring drill bit is typically mounted on the lower end of a drill string and is rotated by rotating the drill string at the surface or by actuation of downhole motors or turbines, or by both methods. With weight applied to the drill string, the rotating drill bit engages the earthen formation and proceeds to form a borehole along a predetermined path toward a target zone. The borehole formed in the drilling process will have a diameter generally equal to the diameter or "gage" of the drill bit.
A typical earth-boring bit includes one or more rotatable cutters that perform their cutting function due to the rolling movement of the cutters acting against the formation material. The cutters roll and slide upon the bottom of the borehole as the bit is rotated, the cutters thereby engaging and disintegrating the formation material in its path. The rotatable cutters may be described as generally conical in shape and are therefore sometimes referred to as rolling cones. Such bits typically include a bit body with a plurality of journal segment legs. The cone cutters are mounted on bearing pin shafts which extend downwardly and inwardly from the journal segment legs. The borehole is formed as the gouging and scraping or crushing and chipping action of the rotary cones remove chips of formation material which are carried upward and out of the borehole by drilling fluid which is pumped downwardly through the drill pipe and out of the bit. The drilling fluid carries the chips and cuttings in a slurry as it flows up and out of the borehole.
The earth disintegrating action of the rolling cone cutters is enhanced by providing the cutters with a plurality of cutter elements. Cutter elements are generally of two types: inserts formed of a very hard material, such as tungsten carbide, that are press fit into undersized apertures in the cone surface; or teeth that are milled, cast or otherwise integrally formed from the material of the rolling cone. Bits having tungsten carbide inserts are typically referred to as "TCI" bits, while those having teeth formed from the cone material are known as "steel tooth bits." In each case, the cutter elements on the rotating cutters functionally breakup the formation to form new borehole by a combination of gouging and scraping or chipping and crushing.
The cost of drilling a borehole is proportional to the length of time it takes to drill to the desired depth and location. The time required to drill the well, in turn, is greatly affected by the number of times the drill bit must be changed in order to reach the targeted formation. This is the case because each time the bit is changed, the entire string of drill pipe, which may be miles long, must be retrieved from the borehole, section by section. Once the drill string has been retrieved and the new bit installed, the bit must be lowered to the bottom of the borehole on the drill string, which again must be constructed section by section. As is thus obvious, this process, known as a "trip" of the drill string, requires considerable time, effort and expense. Accordingly, it is always desirable to employ drill bits which will drill faster and longer and which are usable over a wider range of formation hardness.
The length of time that a drill bit may be employed before it must be changed depends upon its rate of penetration ("ROP"), as well as its durability or ability to maintain an acceptable ROP. The form and positioning of the cutter elements (both steel teeth and TCI inserts) upon the cone cutters greatly impact bit durability and ROP and thus are critical to the success of a particular bit design.
Bit durability is, in part, also measured by a bit's ability to "hold gage," meaning its ability to maintain a full gage borehole diameter over the entire length of the borehole. Gage holding ability is particularly vital in directional drilling applications which have become increasingly important. If gage is not maintained at a relatively constant dimension, it becomes more difficult, and thus more costly, to insert drilling apparatus into the borehole than if the borehole had a constant diameter. For example, when a new, unworn bit is inserted into an undergage borehole, the new bit will be required to ream the undergage hole as it progresses toward the bottom of the borehole. Thus, by the time it reaches the bottom, the bit may have experienced a substantial amount of wear that it would not have experienced had the prior bit been able to maintain full gage. This unnecessary wear will shorten the bit life of the newly-inserted bit, thus prematurely requiring the time consuming and expensive process of removing the drill string, replacing the worn bit, and reinstalling another new bit downhole.
To assist in maintaining the gage of a borehole, conventional rolling cone bits typically employ a heel row of hard metal inserts on the heel surface of the rolling cone cutters. The heel surface is a generally frustoconical surface and is configured and positioned so as to generally align with and ream the sidewall of the borehole as the bit rotates. The inserts in the heel surface contact the borehole wall with a sliding motion and thus generally may be described as scraping or reaming the borehole sidewall. The heel inserts function primarily to maintain a constant gage and secondarily to prevent the erosion and abrasion of the heel surface of the rolling cone. Excessive wear of the heel inserts leads to an undergage borehole, decreased ROP, increased loading on the other cutter elements on the bit, and may accelerate wear of the cutter bearing and ultimately lead to bit failure.
In addition to the heel row inserts, conventional bits typically include a gage row of cutter elements mounted adjacent to the heel surface but oriented and sized in such a manner so as to cut the corner of the borehole. In this orientation, the gage cutter elements generally are required to cut both the borehole bottom and sidewall. The lower surface of the gage cutter elements engage the borehole bottom while the radially outermost surface scrapes the sidewall of the borehole. Conventional bits also include a number of additional rows of cutter elements that are located on the cones in rows disposed radially inward from the gage row. These cutter elements are sized and configured for cutting the bottom of the borehole and are typically described as inner row cutter elements.
Differing forces are applied to the cutter elements by the sidewall than the borehole bottom. Thus, requiring the gage cutter elements to cut both portions of the borehole compromises the cutter element's design. In general, the cutting action operating on the borehole bottom is predominantly a crushing or gouging action, while the cutting action operating on the sidewall is a scraping or reaming action. Ideally, a crushing or gouging action requires a cutter element made of a tough material, one able to withstand high impacts and compressive loading, while the scraping or reaming action calls for a very hard and wear resistant material. One grade of steel or tungsten carbide cannot optimally perform both of these cutting functions as it cannot be as hard as desired for cutting the sidewall and, at the same time, as tough as desired for cutting the borehole bottom. As a result, compromises have been made in conventional bits such that the gage row cutter elements are not as tough as the inner row of cutter elements because they must, at the same time, be harder, more wear resistant and less aggressively shaped so as to accommodate the scraping action on the sidewall of the borehole.
The rolling cone cutters of conventional steel tooth bits include circumferential rows of radially-extending teeth. In such bits, it is common practice to include a gage row of steel teeth employed both to cut the borehole corner and to ream the sidewall. A known improvement to this bit design is to include a heel row of hard metal inserts to assist in reaming the borehole wall. A cone cutter 114 of such a prior art bit 110 is generally shown in FIG. 1 having gage row teeth 112 and heel row inserts 116. As shown, the gage row teeth 112 include a gage facing surface 113 and a bottom facing surface 115 at the tip of the tooth 112. When the cone cutter 114 has been rotated such that a given gage row tooth 112 is in position to engage the formation as shown in FIG. 1, gage facing surface 113 generally faces and acts against the borehole sidewall 5, while bottom facing surface 115 at the tip of the tooth 112 acts against the bottom of the borehole.
Because the tooth 112 works against the borehole bottom, it is desirable that it be made of a material having a toughness suitable of withstanding the substantial impact loads experienced in bottom hole cutting. At the same time, however, a significant portion of the tooth's gage facing surface 113, works against the sidewall of the borehole where it was subject to severe abrasive wear. Because tooth 112 cuts the corner of the borehole and thereby is required to perform both sidewall and bottom hole cutting duties, a compromise has had to be made in material toughness and wear resistance. Consequently, in use, the tooth 112 has tended to wear into a rounded configuration as the portion of the gage facing surface 113 closest to the tip of the tooth 112 wears due to sidewall abrasion and bottom hole impact. This rounding off of tooth 112 has tended to reduce the ROP of the bit 110 and also tended ultimately to lead to an undergage borehole.
More specifically, as gage row teeth 112 begin to round off, the heel row inserts 116 are initially capable of maintaining the full gage diameter of the borehole. However, as the heel inserts are called upon to cut increasingly more and more of the formation material as the teeth 112 are rounded off further, the heel inserts themselves experience faster wear and breakage. Ultimately, the bit's ability to maintain gage is lost.
In prior art bits like that shown in FIG. 1, breakage or wear of heel inserts 116 leads to an undergage condition and accelerates the bit's loss of ROP as described above. This can best be understood with reference to FIGS. 2A-C which schematically shows the relationship of conventional heel insert 116 with respect to the borehole wall 5 as the insert performs its scraping or reaming function. These Figures show the direction of the cutter element movement relative to the borehole wall 5 as represented by arrow 109, this movement being referred to hereinafter as the "cutting movement" of the cutter element. This cutting movement 109 is defined by the geometric parameters of the static cutting structure design (including parameters such as cone diameter, bit offset, and cutter element count and placement), as well as the cutter element's dynamic movement caused by the bit's rotation, the rotation of the cone cutter, and the vertical displacement of the bit through the formation. As shown in FIG. 2A , as the cutting surface of insert 116 first approaches and engages the hole wall, the formation applies forces inducing primarily compressive stresses in the leading portion of the insert as represented by arrow 119. As the cone rotates further, the leading portion of insert 116 leaves engagement with the formation and the trailing portion of the insert comes into contact with the formation as shown in FIG. 2C. This causes a reaction force from the hole wall to be applied to the trailing portion of the insert, as represented by arrow 120 (FIG. 2C), which produces tensile stress in the insert. With insert 116 in the position shown in FIG. 2C, it can be seen that the trailing portion of the insert, the portion which experiences significant tensile stress, is not well supported. That is, there is only a relatively small amount of supporting material behind the trailing portion of the insert that can support the trailing portion to reduce the deformation and hence the tensile stresses, and buttress the trailing portion. As such, the produced tensile stress will many times be of such a magnitude so as to cause the trailing section of the heel inserts 116 to break or chip away. This is especially the case with inserts that are coated with a layer of super abrasive, such as polycrystalline diamond (PCD), which is known to be relatively weak in tension. Breakage of the trailing portion or loss of the highly wear resistant super abrasive coating, or both, leads to further breakage and wear, and thus accelerates the loss of the bit's ability to hold gage.
Accordingly, there remains a need in the art for a steel tooth drill bit and cutting structure that is more durable than those conventionally known and that will yield greater ROP's and an increase in footage drilled while maintaining a full gage borehole. Preferably, the bit and cutting structure would not require the compromises in cutter element toughness, wear resistance and hardness which have plagued conventional bits and thereby limited durability and ROP.